This invention relates to an ignition control device for an internal combustion engine, and particularly, to an improved device for controlling the time period in which an electric current is supplied to an ignition coil.
An induction discharge type ignition device for use in an internal combustion engine is well known, in which high voltage energy is produced in the secondary side of an ignition coil by cutting off the electric current flowing through the primary winding of the coil so that a spark discharge is produced in an ignition plug connected to the secondary winding of the coil.
In general, the high voltage energy mentioned above depends upon the current value of the current flowing through the primary winding of the ignition coil at the time when it is cut off. (The current value will be referred hereinafter as cut-off current.)
Therefore, in order to obtain enough energy to ignite the internal combustion engine, it is necessary to supply an electric current to the primary winding of the ignition coil until the cut-off current becomes enough to ignite. A time from a start of current supply to the primary winding to a time at which the cut-off current reaches a value large enough to ignite, i.e., a current supply time, is determined by the battery voltage, the inductance on the primary side of the ignition coil, and the resistance on the primary side of the coil, etc. Further, the ratio of the current supply time to the ignition period, referred to as a circuit closing ratio hereinafter, depends upon the number of revolutions of the engine. Therefore, the current supply time should be controlled such that a desired cut-off current value is obtained by taking these variables into consideration.
U.S. Pat. No. 4,041,912 and Japanese Kokai No. 40412/1978 disclose control devices capable of controlling the current supply time, respectively. Among others, the device disclosed in the latter controls the current supply time such that the ratio of a time period for which a current in the primary winding of the ignition coil is maintained at a predetermined value to the engine ignition period becomes constant.
However, since, in such conventional device, a calculation of a current supply timing is performed uniformly for every ignition period, a current supply time long enough to ignite cannot be obtained if the ignition period is reduced abruptly due to a high acceleration of the engine or due to large lead angle of ignition timing, in a case where the circuit closing ratio required for a low revolution speed of the engine is small, resulting in a misfiring of the engine.